Since it is no longer possible for flowing water to deactivate a Nether portal, you can't use pistons to push blocks into the nether portal and finally you can't move the obsidian powering the portal with pistons; is it possible to deactivate the portal some other way with redstone?
I'm not too interested in detonating TNT near the portal either, ideally the method needs to be reusable and not destroy the surrounding mechanisms.
Bonus points for activating a Nether portal with redstone as well.
Best Answer
You can put a single bucket of water into a dispenser. When a dispenser "dispenses" an empty bucket, it actually "picks up" a source block in front of it. By putting a single water bucket in a dispenser, when it's activated once it dispenses the water and when it's activated again it removes the water.
You can use this to make a button- or other restone device-triggered "off switch" for a portal. This is a complete reference design:
The trench behind the portal is what keeps the water from flowing everywhere:
To turn it back on, you can build a dispenser with Fire Charges directly against the frame. The Fire Charge goes through the frame, lighting it. The only problem with this is that it consumes a resource that has to be occasionally replenished. Thanks to Resorath for the tip and the video that demonstrates the technique: